Most engines' valve control systems provide fixed duration valve control, which is a compromise between valve timing that is optimal for slow engine speeds and valve timing that is optimal for fast engine speeds. Attempts have been made to provide variable duration valve timing. However, these systems are limited in the amount that the valve timing can be varied.
Some engines use variable lift valves to control engine speed. These are more efficient that engines that use throttles. However, they create a vacuum in the cylinder as the inlet valve is normally only partially lifted on the intake stroke. This vacuum during the intake stroke results in pumping losses in normally aspirated gasoline engines. Diesel engines have a pumping loss inefficiency caused by the fixed length compression, which generally compresses more gases than is required for combustion.
Some engines use turbochargers or superchargers to reduce the vacuum effect during the intake stroke by compressing gases before intake.
Prior art engines generally use springs to close the engine valves, which limits engine speed and requires robust construction of the valve train components.